Consider this scenario: Your 10 year old car has a blown engine and you have to decide whether to have it rebuilt or buy a new car. The rebuilt engine will cost less - about half of the cost of the new car. What should you do?

A new car will be more efficient, but is that the primary factor to consider? Of course not! How about reliability? Is a 10-year-old car with a rebuilt engine likely to last as long as a new car? Even if the engine repair goes well, what about the other systems…transmission, brakes, etc…? When's the next time you'll be in the shop with a major repair?

If you buy a new car, it will probably be more efficient, but is that really the most important factor to consider? Probably not! How about the reliability? Most people don't expect a 10 yr old car with a newly rebuilt engine to last as long as a new car. Most people realize that there is a value that can be ascribed to the enhanced reliability of the new car, even if they can't precisely calculate it.

Extending this analogy to electric motors, how reliable is an aging, but recently repaired motor, compared to a new motor? Intuitively, we expect that the new motor will last longer on average than a repaired motor - but how much longer?

The answer to that question depends on a lot of factors. How traumatic was the failure? Or previous failures if it has been repaired repeatedly? How careful was the repair? Were the burnout temperatures carefully controlled? Was the right gauge wire used in the rewind? Even if it's an IEC motor? (Many shops don't stock metric wire sizes.) Are the core resistances as low as when it was new? Have the air gaps retained their precise tolerances? Are the bearings of the same quality? We know that all of these items can cause increased efficiency loss and therefore more heat generation. And of course heat generation is a primary cause of motor failure.

If a motor repair is done with care and high quality assurance and if the motor has never seen a traumatic failure, then a repaired motor can be returned with efficiency and reliability similar to a new motor. The fact is that most studies have shown that on average repaired motors are returned with somewhat lower efficiencies than when they were new.

We have yet to see good reliability studies of repaired vs. new motors. The only data we have seen is from a study published by Weyerhaeuser about IEEE 841 motors. They found that rewound motors last only half as long as new motors!¹

Our interviews with other manufacturers and from our lab test data lead us to believe that this may be a bit extreme. We are currently projecting rewound motor life at 75% of new motor life. We are trying to validate these projections and we welcome data from any manufacturer who has been tracking this phenomenon.

Why is this important? Consider a plant that operates about 7,000 hrs per year and whose new motors last approximately 7 years. Rewound motors might then be expected to last about 5 and ¼ years. In a population of 100 repaired motors vs. 100 replaced motors operating on this schedule, one should expect almost 5 additional motor failures per year.

If we estimate that the cost of a motor failure is $5,000 per occurrence (including cost of replacement motor, repair labor, and lost productivity,) then the population of 100 repaired motors will cost about $25,000 per year more to operate.

In other words, a new motor will provide a premium of $250 per year in reliability benefits as compared to a repaired motor. Remember, this is a calculation of reliability benefits only and does not include the efficiency benefits that new motors generally have. To arrive at the total benefits of new high efficiency motors, you must add this "reliability premium" to the efficiency savings. This may change the economics of many of your repair/replace decisions.

The bottom line is that if you're not including motor reliability in your motor management decisions you're missing part of the equation. Mr. Raj Kumar of Ralston Purina reported at the 2003 National Manufacturing Week that the reliability benefits of good motor management outweigh efficiency benefits 10 to 1. It's time to look beyond motor efficiency. If you'd like to put this into action, contact us and we'll help you get started.

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¹Holmquist, J.R., "Reasons for Using IEEE Standard 841-1994 Motors for the Forest Products Industry", Proceedings of Pulp and Paper Industry Technical Conference, June 1998, pages 87-95.